Method and apparatus for detection of analyte using a flexural plate wave device and magnetic particles

ABSTRACT

A system for detecting an analyte in a sample includes a resonant sensor that has a sensor surface coated with the capture agent. A plurality of magnetic particles, each of which is coated with a capture agent having an affinity for the analyte, is combined with the sample to form a plurality of analyte-particle complexes. The system also includes a transport arrangement for transporting the sample to the sensor surface, and a magnetic field inducing structure constructed and arranged to establish a magnetic field at and adjacent to the sensor surface. The resonant sensor produces a signal corresponding to an amount of analyte-particle complexes that are bound to the sensor surface.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent Application Ser. No. 60/690,592, filed Jun. 15, 2005.

TECHNICAL FIELD

The present invention relates to sensors for detecting analyte in liquid samples.

BACKGROUND OF THE INVENTION

Significant challenges for a system that detects analytes (e.g., biological agents) in liquid media include concentration of the analyte in the media, and transport of the analyte to a sensor surface. For biological applications, concentration issues generally arise since the concentrations of such analytes tend to be low. Additionally, biological analytes (e.g., cells, cell fragments and macromolecules such as proteins and nucleic acids) tend to be relatively large; hence, transport issues arise because these larger analytes diffuse in fluid solution very slowly. A key metric for competitive detection is the amount of analyte accumulated on a sensor per unit time. For good performance, the rate of accumulation (and the resulting signal transient) needs to be fast relative to the sensor drift rate.

Another key performance metric for an analyte detection system is the degree to which the system can preferentially collect the analyte of interest on the sensor surface. Since many biological samples contain extraneous background components (e.g., other proteins, cells, nucleic acids, dirt), it is necessary to prevent these background components from interfering with the desired measurement. So, a transport method that selectively draws the analyte to the sensor and allows interfering background components to pass by has definite advantages. Such a method used in concert with selective binding of the analyte (e.g., antibody, complimentary DNA strands, etc.) to the sensor surface can deliver high sensitivity measurements for samples with large amounts of extraneous background components relative to the amount of analyte.

Various methods for improving transport of analyte to a sensor surface have been proposed, including filtration, novel flow geometries, acoustic fields, electrical fields (time varying and static) and magnetic fields.

Acoustic excitation has been used to draw cells to field nodes, but it is difficult to use this technique alone to transport material to a surface.

Electrical fields (electrophoresis and dielectrophoresis) have been used to enchance transport but are not universally applicable to all analytes and sample types. They are generally more effective for larger analytes (e.g., cells). Furthermore, the electrical properties of microbes can vary within a given species and strain, making it hard to predict system performance under all intended operating conditions. Sometimes it is necessary to tailor the ionic strength of the sample to improve the performance of the transport. This requirement can conflict with the optimum binding or wash conditions in an assay. Also, electrical fields can dissipate energy and heat conductive fluids (e.g, 0.1 M phosphate buffer solution), which is undesirable since heating can damage the biological analytes.

Immunomagnetic separation (IMS) methods are known in the art for isolating analyte from a sample.

SUMMARY OF THE INVENTION

In the described embodiment of an analyte detection system, an analyte binds to a magnetic particle (e.g., a magnetic bead) to form an analyte-particle complex. The analyte-particle complex is transported and localized onto the surface of a sensing device by applying a gradient magnetic field. The magnetic field induces a polarization in the magnetic material of the particle that is aligned with the local magnetic field lines. The particle experiences a net force in the direction of the gradient, causing the particle to migrate toward regions of higher field strength. The magnetic field distribution is tailored to draw analyte-particle complexes from a sample flow and distribute them across the surface of the sensing device. The extraneous, background components of the sample (e.g., cells, proteins) generally have a much lower magnetic susceptibility as compared to the magnetic particles, and so the magnetic field does not significantly influence them. Hence, only a very small fraction of this background material interacts with the sensor surface.

The sensing device in the described embodiment is a flexural plate wave (FPW) device, which functions particularly well with the magnetic particles for two reasons. First, the presence of the magnetic particles on the surface of the sensing device results in an amplified FPW signal response. The larger combined size and density of the analyte-particle complex yields a larger FPW signal response than the analyte alone. Second, the surface of the sensor in the FPW device consists of a thin membrane that is typically only a few micrometers thick, which allows larger magnetic fields and field gradients to be created at the sensor surface because the field source can be positioned closer to the sample flow. This results in higher fractional capture of the analyte from the sample. With this higher capture rate and efficiency, it is possible to process larger sample volumes in shorter times than would be otherwise possible.

In one aspect, an apparatus for detection of an analyte includes a fluid chamber having at least one opening for fluid to enter, and a flexural plate wave device defining at least a portion of at least one interior surface of the fluid chamber. The apparatus further includes a monitoring device to monitor at least one signal output by the flexural plate wave device, a plurality of magnetic particles coated with a capture agent having an affinity for the analyte, and a first source of magnetic flux to selectively attract magnetic particles to the at least one interior surface of the fluid chamber.

In another aspect, a cartridge for a resonant device system includes a first fluid chamber having at least one opening for fluid to enter, and a flexural plate wave device defining at least one interior surface of the fluid chamber. The apparatus further includes a first source of magnetic flux to selectively attract magnetic particles to the at least one interior surface of the first fluid chamber.

In another aspect, a method for detection of an analyte includes combining a fluid containing an analyte with a plurality of magnetic particles that comprise a capture agent having an affinity for the analyte to produce at least some magnetic particles bound to at least some analyte. The method further includes directing the combined fluid into a first fluid chamber, wherein at least one surface of a flexural plate wave device is in fluid communication with the fluid in the first fluid chamber. The method also includes creating a first magnetic flux in proximity to the flexural plate wave device to magnetically attract at least some of the bound magnetic particles to the at least one surface of the flexural plate wave device.

In another aspect, a method for detection of an analyte includes coating at least a portion of a surface of a flexural plate wave device located in a fluid chamber with a first capture agent, and directing a fluid containing an analyte into the fluid chamber to bind some of the analyte to the capture agent located on the flexural plate wave device. The method further includes directing a fluid containing a plurality of magnetic particles that comprise a second capture agent into the fluid chamber, and creating a magnetic flux in proximity to the flexural plate wave device to attract at least some of the magnetic particles towards the surface of the flexural plate wave device.

BRIEF DESCRIPTION OF DRAWINGS

The foregoing and other objects of this invention, the various features thereof, as well as the invention itself, may be more fully understood from the following description, when read together with the accompanying drawings in which:

FIG. 1A shows one embodiment of an analyte detection system, constructed according to the invention.

FIG. 1B shows another embodiment of a portion of the analyte detection system shown in FIG. 1A.

FIG. 2 shows a more detailed view of the FPW sensor shown in FIG. 1A.

FIG. 3 shows a general detection protocol for using an FPW sensor in combination with analyte-particle complexes for detection of biological analytes.

FIG. 4 shows the change in the signal from multiple FPW sensors as a function of time for an exemplary detection protocol.

FIG. 5 is a summary plot showing the final signal change detected as a function of original analyte concentration.

FIG. 6 shows a time evolution plot for a detection protocol, similar to the plot shown in FIG. 4, but rather for a PSA analyte.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of an analyte detection system 100, constructed according to the invention, is shown in FIG 1A. The system 100 includes a network of channels 102 for transporting various test solutions (also referred to herein as “test fluids” or “fluids”) through an FPW device 104. The following U.S. patents and patent applications, all of which are hereby incorporated by reference, describe examples of the various types of FPW devices suitable for use in the present invention: U.S. Pat. No. 5,129,262, U.S. Pat. No. 5,189,914, U.S. Pat. No. 6,688,158 B2, U.S. patent application Ser. No. 10/324,685, U.S. Pat. No. 5,668,303, U.S. Pat. No. 5,836,203, and U.S. Patent Application 20040038195.

For example, U.S. Pat. No. 5,129,262 describes an ultrasonic sensor that has a thin planar sheet of material forming a Lamb wave propagation medium. Lamb waves, also known as plate-mode waves, can propagate only through a material of finite thickness. In contrast to surface acoustic waves (SAWs), which require a propagation medium having a thickness on the order of hundreds of times the wavelength of the propagating SAW, Lamb waves require a propagation medium which is at most only several wavelengths thick, and typically only a fraction of the wavelength of the propagating Lamb wave. The thickness of the sheet is no greater than about twenty microns. A Lamb wave generator generates Lamb waves in the planar sheet, and an output device produces an electrical signal that represents the propagation characteristics of the Lamb waves propagating along the sheet. A measuring device measures selected characteristics of the output electrical signal. The planar sheet has some physical characteristics that depend upon the value of a measurand acting on the sheet, and those physical characteristics consequently determine the propagation characteristics of the Lamb waves that propagate along the sheet. Since the electrical signal from the output device represents the propagation characteristics, the electrical signal also represents the value of the measurand acting on the sheet.

The Lamb wave device described in U.S. Pat. No. 5,129,262 can be employed, for example, in biological sensing. The planar sheet described above can be pre-coated with antibody molecules, so that the frequency of the device changes upon immersion in or contact with a liquid that contains the corresponding antigen. Antigen-antibody attachment at the surface of the propagation medium acts to alter the wave velocity of the Lamb waves in the sheet. The change in wave velocity causes the oscillation frequency to change in a delay line oscillator form of the device. Also, the sheet may be made of a porous and permeable material, allowing the coating of antibody molecules over a greater surface area of the sheet and also allowing the antigen-containing liquid to be flowed through the membrane, in order to speed up the antigen-antibody attachment. Other biological interactions may also be sensed, and additional applications include immunoassay, clinical laboratory testing, in vivo biomedical monitoring, and biomedical research.

The test solutions used in the described embodiment, for example a blocking solution 106, a sample 108, and a buffer 110, are sourced from reservoir containers 112. The channel path from each of the reservoirs 112 is gated with a valve 114 to control the flow of a particular test solution to a combination point 116 leading to an entry port 118 of the FPW device 104. The test solution flows through the FPW device 104 and exits via an exit port 120, which leads to a pump 122. The pump 122 draws the test solution through the network of channels 102 and through the FPW device 104, and directs the test solution to a waste receptacle 124.

FIG. 1B shows another embodiment of the analyte detection system 100. This embodiment packages the FPW device 104 and its associated fluid chamber 160 as a cartridge 103, i.e., a consumable component that can be removed and replaced. Some embodiments may include a fluid control device 101 such as a plug, obstruction or baffle that alters the flow through the device 104. In one embodiment, the fluid control device 101 operates to cause the fluid flow through the device 104 to pass closer to the sensor surface 143 than if the fluid control device 101 was not present. Further, the source of input test solutions is shown as an input fluid chamber 105 that has an outlet 107 for directing the test solutions into the inlet 109 of the cartridge 103. In some embodiments, the magnetic particles are initially located in the input fluid chamber 105 and the fluid containing analyte is mixed with the magnetic particles in the input fluid chamber 105 and then directed into the cartridge 103 in which the FPW device 104 is located. The magnetic particles may be combined within the input fluid chamber 105 with the fluid containing analyte by a device (e.g., by the action of a pump or a magnetic agitator). FIG. 1B further shows an output fluid chamber 111 with an inlet 113 that receives fluid from the outlet 115 of the cartridge 103. This output fluid chamber 111 may include one or more of the fluid control devices described herein, and it may include one or more mechanisms for storing and/or treating waste fluid.

In at least one embodiment, the junction where the outlet 107 of the input fluid chamber 105 meets the inlet 109 of the cartridge 103 is constructed and arranged to allow repeatable connection and disconnection. Similarly, the junction where the outlet 115 of the cartridge 103 meets the inlet 113 of the output fluid chamber 111 is constructed and arranged to allow repeatable connection and disconnection. In some embodiments, these junctions are constructed and arranged to require tools for connection and disconnection, such as threaded couplings that require a wrench or other such tool to affect the coupling and decoupling. In other embodiments, these junctions are constructed and arranged to allow quick and easy manual connection and disconnection, without any extra tools or accessories. Such couplings, both requiring and not requiring tools, are known in the art. In some embodiments, there are multiple input fluid chambers and output fluid chambers. In some embodiments, one or more input and/or output fluid chambers are part of the cartridge 103. Further, in some embodiments, one or more sources of magnetic flux are part of the cartridge.

The FPW device 104 is shown in more detail in FIG. 2. In an FPW device 104, strain energy is carried in bending and tension in the device. In some embodiments, it is desirable for the thickness-to-wavelength ratio of the FPW device 104 to be less than one, and in some cases much less than one. In general, the wavelength “λ” of the FPW device 104 is approximately equal to the pitch of the interdigitated electrodes as described herein. In one embodiment, the thickness-to-wavelength ratio of the FPW device 104 is 2 μm/38 μm. In other embodiments, the FPW device 104 is designed to isolate a particular mode (e.g., any mode from the zero^(th) order mode to higher order modes) or bandwidth of modes associated with the device. For example, an FPW device 104 having a thickness/wavelength of 2 μm/38 μm as described above would isolate the 80^(th) mode of the FPW device 104. The FPW device 104 can be designed to achieve this effect by selecting a particular pattern for the interdigitated electrodes deposited on the device. In one embodiment, the FPW device 104 is rectangular in shape. The FPW device 104 can, alternatively, be circular or elliptical, or some other planar shape.

In general, the FPW device 104 is constructed from a silicon wafer 130, using micro-fabrication techniques known in the art. In the described embodiment, a cavity 132 is etched into the wafer 130 to produce a thin, suspended membrane 134 that is approximately 1.6 mm long, 0.3 mm wide and 2 μm thick. The overall wafer 130 thickness is approximately 500 μm, so the depth of the cavity 132 is just slightly less than the wafer 130 thickness. A 0.5 μm layer 136 of aluminum nitride (AIN) is deposited on the outer surface (i.e., the surface opposite the cavity 132) of the membrane 134, as shown in the expanded view insert of FIG. 2. Two sets of inter-digitated metal electrodes 138 are deposited upon the AlN layer. A thin layer 140 of gold (approximately 500 angstroms) is deposited on the inner surface (i.e., the surface facing the cavity 132) of the membrane 134 to facilitate immobilization of capture agents (described in more detail below).

In operation, instrument/control electronics 126 (referring to FIG. 1A) apply a time-varying electrical signal to at least one set of electrodes 138 to generate vibrations in the suspended membrane 134. The instrument/control electronics 126 also monitor the vibrational characteristics of the membrane 134 by receiving a sensor signal from at least a second set of electrodes 138. When liquid is in contact with the cavity side 132 of the membrane 134, the maximal response of the plate structure is around 15-25 MHz. The instrument/control electronics 126 compare a reference signal to the sensor signal from the second set of electrodes to determine the changes in the relative magnitude and phase angle of the sensor signal as a function of frequency. The instrument/control electronics 126 interpret these changes to detect the presence of the targeted analyte. In some embodiments, the instrument/control electronics also determines, for example, the concentration of the targeted analyte on the inner surface of the membrane 134.

Capture agents targeting the analyte of interest are immobilized on the thin layer of gold 140 covering the inner surface of the membrane 134, as described above. Thiol-terminated alkyl chains are linked to the gold surface forming a self-assembled monolayer (SAM). A fraction of the SAM chains are terminated with reactive groups (e.g, carboxyl) to allow covalent linking of capture agents to the SAM chains using biochemical process steps known in the art. The remainder of the SAM chains are terminated with non-reactive groups, preferably ones that have a hydrophilic character to resist nonspecific binding (e.g., oligomers of ethylene glycol). Other surface chemistries are described in the literature and can be used to produce a capture surface.

The FPW device 104 is packaged to allow electrical connections to the electrodes 138 on the outer surface of the membrane 134. Additionally, the FPW device 104 is mechanically supported by a channel block 142, to allow for the inner surface of the membrane 134 to contact the test solutions and an interface is provided for contacting the sensor surface 143 with the liquid sample. The channel block 142 creates a path (fluid chamber 160) for the test solutions to flow from an input port 118, past the inner surface of the membrane 134 and then out of an exit port 120. A seal 144 is formed between the FPW device 104 and the channel block 142 to prevent test solutions from escaping from the channels 102 formed within the combination of the FPW device 104 and the channel block 142. The channel block 142 thus forms a fluid chamber, of which the FPW device 104 comprises one of the interior walls.

The channels 102 through the combination of the FPW device 104 and the channel block 142 are approximately 0.5 mm in diameter. The channel block 142 can be formed from a variety of materials, including plastic, metal or ceramic, among other materials.

The system 100 includes one or more fluid control devices for changing at least one fluid property, such as flow, pressure, or trajectory to name a few, within the system 100. The pump 122 and valves 114 shown in FIG. 1A that direct and control the flows of various test solutions through the device and over the sensor surface 143 (as required to execute a test protocol) are all examples of fluid control devices. In general, a fluid control device changes the at least one fluid property in the vicinity of at least one surface within the fluid chamber 160 of the device 104. Generally, this is done to distribute the magnetic particles along at least a portion of the sensor surface 143. As described above, in some embodiments the fluid control device is a pump (e.g., a peristaltic pump, centrifugal pump, rotary pump, electro-osmotic pump). In some embodiments, the pump is located on the entrance side of the fluid chamber, and in other embodiments the pump is located on the exit side of the fluid chamber. In some embodiments, the device is a flow diverter (e.g., a plug, obstruction wall or baffle) that is disposed relative to the fluid chamber to alter the fluid flow in the vicinity of the at least one interior surface of the fluid chamber.

Referring to FIG. 1A, a single pump 122 is positioned on the waste side of the FPW device 104. Suction that the pump 122 generates draws buffer 110 or analyte in the sample 108 from their respective reservoir containers 112 on the supply side of the FPW device 104. Valves 114 are positioned on the supply side of the device 104 to control which test solution is directed over the sensor surface 143 at any time during the test protocol. The pump 122 controls the flow rate of the test.

A device for regulating temperature (e.g., a thermoelectric cooler) may be associated with the FPW device 104 and channel block 142. This reduces the impact of variable environmental conditions on the FPW device 104 output by maintaining the device 104 at a relatively constant, known temperature. In an alternative embodiment, a temperature sensor is included within the system 100, for example as part of the FPW device 104. The sensor signal from the FPW device 104 is scaled, at a specific instant in time (or during a period of time), based on the output of the temperature sensor, in order to produce a signal that is independent of the effects of temperature variations. This scaling could be done based on a mathematical model, or an analytical model, or some hybrid combination of a mathematical and analytical model.

In some embodiments of the system 100, a filter is included in the path of the test solution to selectively filter particles (e.g., magnetic particles and biological materials) of a particular size to prevent them from entering the fluid chamber. By way of example, a particular testing protocol may include steps for changing the filter during the test. This would allow different types (i.e., sizes) of analytes and magnetic particles to be directed into the fluid chamber, and thereby tested by the system 100, during different portions of the test.

Magnetic particles (e.g., paramagnetic or super-paramagnetic beads or microspheres), which have their surfaces coated with a capture agent, are mixed with a sample containing the analyte. After a prescribed mixing time analyte-particle complexes 146 result as do particles 147 that have bound nonspecific materials and particles 148 that have bound nothing. The particles 146, 147 and 148 are located in the sample reservoir 112.

The system 100 further includes a magnetic field inducing structure 150 for producing magnetic flux in the vicinity of the membrane 134. In FIG. 1A, the source of magnetic flux is a retractable magnet 150 arranged to normally be in close proximity to the membrane 134 of the FPW device 104. When the magnet 150 is in close proximity to the membrane 134, the magnet 150 produces a significant gradient magnetic field in the vicinity of the membrane 134. Under control of the instrument/control electronics 126, the retractable magnet 150 can be retracted away from the membrane 134 by a distance sufficient to substantially reduce magnetic fields in the vicinity of the membrane 134. In one embodiment, when in close proximity to the membrane 134, the magnet 150 is situated approximately 200 μm from the sensor surface 143 of the membrane 134. In another embodiment, when in close proximity to the membrane, the magnet 150 is situated between about 50 μm to about 100 μm from the sensor surface 143 of the membrane 134.

When the magnet 150 is in close proximity to the membrane 134, the magnet 150 provides a source of magnetic flux to draw the magnetic particles from the sample to the sensor surface 143. The analyte-particle complexes 146, as well as particles 147 with nonspecifically bound material and particles 148 with nothing bound, migrate from the liquid sample until they encounter the sensor surface 143. The analyte binds with the capture agent on the sensor surface 143. Thus, the analyte forms a link between the magnetic particle and sensor surface. The particles 147 with non-specially bound material and particles 148 with nothing bound are held at the sensor surface 143 by the magnetic field. Additionally, weak binding forces can act between the particles 146, 147, and 148 and the sensor surface 143. During the wash step of the protocol (described in more detail below), the magnet 150 is retracted to reduce the magnetic force experienced by the particles that have accumulated at the sensor surface 143. The wash flow rate is increased to remove particles 147 and 148 that are not bound to the surface by analyte. Since the particles 147 with nonspecifically bound material as well as particles 148 with nothing bound are more weakly linked to the sensor surface 143 than the analyte-particle complexes 146, they release from the sensor surface 143 at a lower wash flowrate (and corresponding hydrodynamic force). Hence, removing the magnet 150 (i.e., substantially reducing the magnetic force experienced by the particles 146, 147, and 148 at the sensor surface 143) is used to distinguish between particles with analyte 146 from those without (particles 147 and 148). One technique for engaging and retracting the magnet 150 is to mount it on a carriage (not shown) that is actuated by a cam system (not shown).

The magnet 150 material, geometry and distance from the sensor surface 143 determine the field shape and field gradient, and therefore, the force that the analyte-particle complexes 146 experience. High strength permanent magnets for use as the retractable magnet 150 are available commercially. For example, 1 mm diameter cylindrical NdFeB magnets can be purchased from several vendors (e.g., Dexter Magnetic Technologies). In one embodiment, a 1 mm diameter and 5 mm long NdFeB magnet 150 is positioned within 0.1 mm of the sensor surface 143 when engaged. When retracted the magnet 150 is at least 0.5 mm from the sensor surface 143. Since the membrane 134 of the FPW device 104 is very thin (2 μm) and made of nonmagnetic materials (e.g., silicon, aluminum nitride or gold), the membrane 134 does not significantly perturb the magnetic field on the sensor surface 143 side of the device 104. As a result, very high magnitude magnetic fields and large field gradients can be achieved, as is necessary for high collection efficiencies.

The sample flow rate through the channels 102 is determined (e.g., specified by an operator) by the residence time necessary for good collection efficiency. The sample flow rate is adjusted so that the average velocity over the sensor surface 143 is between about 1 and about 5 mm/s. With an iron oxide paramagnetic particle with a diameter of approximately 3 μm, collection efficiencies approaching 50% can be achieved.

Other configurations of the source 150 of magnetic flux (i.e., the magnet) may be used. For example, an electromagnet can be used instead of a permanent magnet. The electromagnet includes pole pieces that extend to focus the field flux near the sensor surface 143 of the device 104.

Alternatively, a magnetizable material can be fashioned and positioned adjacent to the sensor surface 143 (within 0.1 mm), and a separate magnet combined with an open face of the magnetizable material to induce a magnetic field in the magnetizable material. The magnetic field induced in the material serves to locate desirable field gradients near the sensor surfaces 143. In this way, large, low cost magnets can be used, and a single magnet can be used to address multiple sensors, depending on the fashioning of the material. Examples of useful materials for this purpose are pure iron, high mu metals such as alloy 49 (high nickel content iron), sna silicon steels (1-2% silicon typical). An advantage of using such a magnetizable material with an associated magnet is that the sensor assembly can be simplified, allowing lower cost manufacturing. A low precision actuator can be used for engaging and retracting the magnet since the magnet need only contact the ferromagnetic core or be fully withdrawn. In the described embodiment where the magnet 150 is positioned in close proximity to the sensor surface 143, a higher level of precision is required to achieve good assay repeatability. Although there is some loss of field strength with this approach, it is still possible to design the overall system to achieve good capture efficiencies (e.g., >10%).

The shape of the tip of the field inducing structure (e.g., magnet or ferromagnetic material) may be tailored to enhance and/or concentrate the field gradient at the surface. Since the size of the FPW device 104 (e.g., 0.3 mm×1.6 mm) is typically smaller than conventionally formed magnets or machined inductors, the portion of the field inducing structure adjacent to the membrane 134 can be tapered to concentrate the magnetic field in one or more locations on the sensor surface 143. Tapering the tip acts to increase both the local field magnitude and the local field gradients. For example, a wedge-shaped tip is well suited to the current FPW device geometry.

One embodiment of the system 100 includes an optional second source 150 a of magnetic flux that opposes or partially opposes the first source 150 of magnetic flux. This second source 150 a of magnetic flux can be used to dislodge some of the magnetic particles that have adhered to the sensor surface 143. It may, for example, dislodge magnetic particles 148 that do not have any bound analyte; they would not be as strongly attached to the sensor surface 143 as the particles 146 that do have bound analyte. In some embodiments, the first source 150 of magnetic flux is turned off or moved away from the sensor surface 143 and then, the second source 150 a of magnetic flux is positioned relative to the at least one surface of the fluid chamber to selectively remove magnetic particles. This may be done, for example, to remove magnetic particles 148 that do not have any bound analyte and therefore they are not as strongly bound to the sensor surface 143. This would achieve a similar effect as increasing the flow of fluid to remove magnetic particles 148 that do not have any bound analyte.

Controlling the distribution of the analyte-particle complexes 146 on the surface 143 of the device 104 can improve the device performance, since the device 104 has a suspended membrane 134 and not all parts of the membrane 134 contribute equally to the moving mass of the detectable resonance. For example, the system 100 can be constructed and arranged to distribute the analyte-particle complexes 146 within one third of the FPW device 104 width along the middle two-thirds of the centerline of the long axis of the membrane 134. Taking into account flow field effects, the shape of the tip of the field-inducing structure (e.g., magnet 150) can be such that the field magnitude and field gradient increase in the direction of the flow over the sensor membrane 134. That is, analyte-particle complexes 146 in the downstream regions, where the boundary layer is partially depleted of analyte, experience a higher field and field gradient than do analyte-particle complexes 146 in the upstream regions.

In general, the system 100 can be constructed and arranged to concentrate magnetic particles in one or more particular regions of the sensor surface 143. The response of the device 104 may not be uniform over the sensor surface 143 due to characteristics of the fabrication materials or the specifics of the sensor design. Thus, high sensitivity regions of the device 104 may be non-uniform and asymmetrical with respect to the long and short axis centerlines of the device 104. Thus, the tip of the field inducing structure may be shaped to concentrate magnetic particles in the region or regions of highest sensitivity.

Varying the flow rate through the device 104 can also be used to achieve a more uniform coverage of analyte-particle complexes 146 for a given magnetic field distribution. For a given field, magnetic particles interact with the sensor surface 143 as determined by the bulk fluid flow rate, much like a ballistic object might fall in the presence of the gravity body force. In this case, however, the magnetic induced force dominates. By varying the flow rate, the analyte-particle complexes 146 can be caused to interact with the sensor surface 143 at substantially different locations along the stream-wise flow direction. Furthermore, as the magnetic particles pile up (a non-desirable occurrence if they are to be exposed to the sensor surface 143) the flow can be reversed and subsequently pulsed forward in order to pull the pile over and thus communicate more particles with the sensor surface 143. In one embodiment of the system 100, selective location of the magnetic particles along the sensor surface 143 is achieved by selectively altering, over the course of the detection protocol, either one or both of the magnetic flux source and the property or properties of the fluid flow along the sensor surface 143.

One embodiment of the system 100 includes a device (e.g., optical, magnetic) for characterizing at least one property of the magnetic particles that are attached or attracted to the sensor surface 143. This device could be an integral part of the FPW device 104, or it could be a part of the magnet 150, or it could be a discrete component apart from other components of the system 100. Such a device may be used to detect the presence of the particles, and also to determine parameters related to the particle, for example, the size, quantity, concentration, or density of the particles that are attracted to the sensor surface 143.

One embodiment of the system 100 includes an identification device for allowing an operator or computer to identify the system 100 or a particular component of the system for tracking usage of the system or component. The identification device may include a symbol or image such as a bar code, an identification number, or other identifying mark. The identification device may include an actual component, passive or active, such as an RFID tag, an integrated circuit or other such component known in the art for providing identifying information. Many such devices are known in the art, although any contemplated identification device may be used.

A general detection protocol 200 for using an FPW device 104 in combination with analyte-particle complexes 146 for detection of biological analytes is shown in FIG. 3.

The first step 202 of the detection protocol 200 is acquiring and preparing 202 the analyte sample. Various preparation processes may need to be performed prior to testing, depending upon the particular type of analyte being tested. For example, to detect microbes in food (e.g., E. coli in ground beef), a sample would be first mixed with enrichment broth, stomached, incubated and filtered. For detecting proteins in blood, the sample would first be filtered or centrifuged and the serum separated. Specific examples of test protocols that include sample preparation steps are described herein.

The next step 204 of the detection process is mixing affinity-coated paramagnetic particles (i.e., beads) with the prepared analyte sample. Paramagnetic or super-paramagnetic particles are available commercially from a number of vendors (e.g., Dynal Biotech, Oslo, Norway). Typical diameters range from 50 nm to 10 μm, and such particles can be procured already coated with affinity agents (e.g., antibodies, proteins, and nucleic acid probes) targeting a variety of analytes (e.g., cells, proteins and nucleic acids). Alternatively, the particles can be purchased with various reactive chemical groups (e.g., epoxy, carboxyl, and amine) in order to attach a capture agent of choice. Standard biochemical protocols are available for this purpose.

The sample with paramagnetic particles added is agitated 206 for an amount of time determined by the particular analyte and capture agent. During this process, the particles bind with analyte so that the analyte is captured on the particles. In some cases, the sample can be tested directly at this point. But, in other cases it is advantageous to perform separation steps 208 to isolate the analyte bound to the particles from the rest of the original sample. These separation steps 208 reduce interference from other biological material in the assay. Manual or automated equipment for performing such separation steps is available commercially (e.g., Dexter Magnetic Technologies, Dynal Biotech). The basic process uses a magnet to localize the paramagnetic particles on a surface so that the majority of the sample liquid can be aspirated. The magnet (e.g., magnet 150 of FIG. 1A) is then removed, and clean buffer solution is added to re-suspend the particles.

A baseline step 210 is executed prior to testing the processed analyte sample with the FPW device 104. During the baseline step 210, a reference solution 106 is flowed through the system to rinse/block the sensor 104, and the instrument/control electronics 126 excite the device 104 and records the resulting initial baseline signal from the device 104.

A sample transport step 212 follows the baseline step 210. The sample 108 containing the analyte-particle complexes 146 is flowed over the sensor surface 143 with the magnet 150 engaged. Analyte-particle complexes 146 are collected on the sensor surface 143. After a prescribed volume of sample 108 has flowed through the device 104, the magnet 150 is retracted to release the particles 147 and 148 from the sensor surface 143 that do not have bound analyte, and the flow is switched to a wash solution (e.g., buffer solution 110). The flow rate of the wash solution is increased to help remove loosely bound particles 147 and 148, as well as other material in the sample that may have bound to the sensor surface 143

An acquisition step 214 follows the sample transport step 212. Reference solution 106 is again run through the device 104, and the instrument/control electronics 126 excite the device 104 to acquire and record a final baseline signal from the device 104.

The system 100 determines the amount of analyte accumulated on the sensor surface 143 during the transport step 212 by comparing 216 the initial baseline signal and the final baseline signal, which correspond to the vibrational characteristics of the FPW device 104 in the reference solution before and after, respectively, the acquisition step 214. Analyte-particle complexes 146 bound to the sensor surface 143 change the vibrational characteristics of the FPW device 104, and the amount of change to the vibrational characteristics correspond to the amount of anaylyte-particle complexes bound to the sensor surface 143.

FIG. 4 shows the change in the signal from multiple FPW devices 104 as a function of time for an exemplary detection protocol. The square symbols correspond to a device 104 exposed to an analyte; the triangles and stars correspond to negative controls (in which there is no analyte on the beads). The data shown in FIG. 4 (and FIG. 5 as described below) represent an exemplary detection protocol for which the analyte is E. coli bacteria, or generally a cellular analyte.

In this particular experiment, the frequency of a resonant peak was tracked. FIG. 4 shows that the resonant frequency of the FPW device 104 decreases as the magnetic particles 146, 147 and 148 accumulate on the surface. Once the magnet 150 is removed and some of the magnetic particles are washed away, the frequency of the device 104 increases. Eventually, the system establishes a final baseline that can be compared to the initial baseline taken at the start of the test, or to that of a control.

FIG. 5 is a summary plot showing the final signal change detected as a function of original analyte concentration.

Other detection protocols may be used with the system 100 described above. Individual steps can be eliminated or added depending on the requirements of a specific application or analyte. For example, FIG. 6 shows a time evolution plot for a detection protocol, similar to the one shown in FIG. 4. FIG. 6, however, depicts a detection protocol for a PSA assay, which demonstrates the capability of the system 100 for detecting proteins. Also, the flow direction can be reversed during the protocol. For example, reversing the flow direction may be useful for washing nonspecifically bound material from the device, or for making more efficient use of the available sample.

Another variation in the detection protocol includes alternating between wash steps and binding steps. This can allow better use of the dynamic range of the device. In come cases, a large fraction of the particles do not have bound analyte, especially at low analyte concentrations. By repeatedly binding and washing away particles, it is possible to accumulate more analyte-particle complexes 146 and, hence, improve the sensitivity of the measurement.

Changing or manipulating the magnetic field distribution at the sensor surface 143 during the transport step 212 can enhance the probability that the analyte attached to a particular particle encounters the sensor surface 143. For example, if the spatial distribution of the field is alternated during binding, it is possible to cause the paramagnetic particles at the sensor surface 143 to roll. In some embodiments, by controlling the spatial distribution of the field, an operator or the instrument/control electronics 126 can be used to control the rolling of the paramagnetic particles along the sensor surface 143.

As described above, introducing a second magnetic field (i.e., a second source of magnetic flux) in the system 100 can improve the control of the assay conditions and enhance the specificity of the assay. For example, during the binding or wash steps of the protocol as described above, applying a secondary magnetic field to the sensor surface 143 can act to pull off weakly bound magnetic particles. The strength of the secondary field can be tailored to generate a force on the analyte-particle complexes 146 at the sensor surface 143 that is below the binding force of the specifically bound analyte but above the typical binding force for nonspecifically bound material. This sequence of steps can be repeated multiple times during the assay to further increase the specificity of the test.

The relative binding strength of the various analyte-particle complexes 146 on the sensor surface 143 can be determined by increasing (continuously or discretely) this magnetic pull-off force during the wash step, while monitoring the response of the FPW device 104. Information on the relative binding strength can be used to distinguish between different analytes in the sample 108.

The particular way the sample interfaces with the device 104 can be different in other embodiments. In the above-described embodiments, the system 100 flows the sample through a channel to establish contact between the analyte-particle complexes 146 and the sensor surface 143. In an alternative variation of the system 100, the FPW device 104 is mounted on a probe and at least partially immersed into a test solution containing magnetic particles bound to an analyte. For this embodiment, the immersion is sufficient to place the bound magnetic particles in proximity to the sensor surface 143 so that the particles are attracted toward the sensor surface 143 and subsequently detected, as described herein. To obtain a baseline signal, the device 104 (or cartridge 103) is immersed in a reference test solution. In some embodiments, a portion of the device 104 (e.g., the membrane 134) is mounted to a probe. Further, only part of the sensor surface 143 of the membrane 134 is placed in contact with the solution containing magnetic particles bound to an analyte. In these embodiments, immersion or controlled movement of the probe in the fluid is sufficient to place the bound magnetic particles in proximity to the sensor surface 143 so that the particles are attracted toward the sensor surface 143 and subsequently detected.

Another alternative embodiment of the system 100 involves mounting the device 104 inside of a tube that can be partially immersed into a well holding the sample 108, and then retracted. A pump applies suction to draw sample into the tube and over the sensor surface 143 (or cartridge 103) when the sample is immersed. The sample is then ejected back into the well by reversing the pump or simply by venting the tube. This cycle of drawing and releasing the sample can be repeated to improve the collection efficiency and, therefore, the performance of the assay.

The following examples illustrate, for one embodiment of the system 100 described herein, steps for preparing and utilizing the system 100 for detecting an analyte.

A. Generalized Method for Capture Agent Functionalization of a Surface of a Flexural Plate Wave Device

-   -   1. Deposit gold onto the surface (e.g., sensor surface 143) of         the flexural plate wave device 104 and clean the gold surface         143 with, for example, oxygen plasma.     -   2. An ideal surface chemistry for the surface 143 of the gold is         one that provides 1) non-specific binding resistance and 2)         reactive groups located on the surface for covalent attachment         of capture agents. An exemplary surface chemistry for the         surface 143 of the gold is a self-assembled monolayer (SAM) of         alkane thiols. The SAM can be formed from a mixture of two         alkane thiols; one terminated with a reactive group for         subsequent covalent attachment of capture agents, and one         terminated with a non-reactive group. By way of example, a         mixture of EG₃-OH (EG3) and EG₆-OCH₂COOH (EG6) terminated         C₁₁-alkane thiols may be used for this purpose. In one         embodiment, the flexural plate wave device 104 (particularly the         surface 143 of the device 104) is placed in contact with the         alkane thiol solution and allowed to incubate at room         temperature for, for example, about 16 hours. The surface 143 of         the flexural plate wave device 104 is then rinsed with ethanol         and blown dry with nitrogen.     -   3. The next step involves covalent attachment of capture agent         to the surface 143 of the flexural plate wave device 104. A         number of methods may be used for covalent attachment of capture         agent. An exemplary method involves covalently linking a biotin         linker moiety to the SAM, then binding a biotinylated antibody         to the flexural plate wave device surface 143 via a streptavidin         linking layer.     -   4. After biotinylating the surface 143 of the flexural plate         wave device 104, the device 104 is functionalized with specific         capture agents through a neutravidin-binding layer. The         functionalization is accomplished by sequentially exposing the         surface 143 of the biotinylated flexural plate wave device 104         to a concentrated neutravidin solution and then to a solution of         the biotinylated capture agent of choice.

B. Detecting E. coli 0157:H7 in Ground Beef Using, for Example, the Method of FIG. 3 (FIG. 5 Contains Data Representative of Various Concentrations of E. coli).

-   -   1. Prepare an analyte sample containing E. coli 0157:H7 with a         concentration greater than about 100 cfu/mL.     -   2. Concentrate the analyte sample in solution by performing an         immunomagnetic separation. A variety of commercial instruments         (e.g., Pathatrix by Matrix Microsciences and Bead Retriever by         Dynal Biotech) or manual methods may be used to perform the         immunomagnetic separation. An exemplary manual method involves:         -   a. Resuspend magnetic beads coated with E. coli antibody             (e.g., Dynabeads anti-E. coli 0157, available from Dynal             Biotech) until the magnetic bead pellet in the bottom of the             tube disappears. Place a microcentrifuge tube in the rack             (e.g., a Dynal MPC-S) of a magnetic plate. Pipette 1-20μL of             magnetic bead stock solution into the tube (the volume of             magnetic bead stock selected is based on desired final bead             concentration).         -   b. Add 1 mL of the analyte sample to the tube in the rack of             the magnetic plate and close the tube.         -   c. Invert the rack a few times. Incubate the solution in the             tube at room temperature for 10 to 60 minutes with gentle             continuous agitation to prevent magnetic beads from             settling.         -   d. Invert the rack several times to concentrate the magnetic             beads into a pellet on the side of the tube. Allow about 3             minutes for proper recovery.         -   e. Open the tube and carefully aspirate and discard the             sample supernatant as well as the remaining liquid in the             tube's cap.         -   f. Remove the magnetic plate.         -   g. Add 1 mL of wash buffer (PBS-Tween) to the tube. Close             the cap of the tube and invert the rack a few times to             resuspend the beads.         -   h. Repeat steps d-g twice.         -   i. Mix the contents of the tube briefly using a vortex             mixer.     -   3. Detection of E. coli 0157:H7         -   a. Functionalize (similarly as described by the steps in A.3             and A.4) the surface 143 of a flexural plate wave device 104             with E. coli 0157:H7 antibody.         -   b. Place a first inlet hose into a tube containing standard             wash buffer (1xPBS with 0.05% Tween 20) and a second inlet             hose into the tube containing the magnetic beads bound             with E. coli 0157:H7 (prepared in B.2). The first inlet hose             and second inlet hose are joined by a t-joint so the two             hoses are in fluid communication and fluid from either the             first inlet hose or from the second inlet hose is directed             to an inlet of the fluid chamber 160. Each of the two hoses             has a valve that is capable of permitting or limiting the             flow of fluid through respective hoses.         -   c. Place a first outlet hose in fluid communication with an             outlet of the fluid chamber 160. Fluid from the first outlet             hose is collected in a waste collection bottle.         -   d. A baseline output signal is obtained using the flexural             plate wave device. The baseline signal is measured with             standard wash buffer flowing into the fluid chamber 160 and             out of the fluid chamber 160 for about 5 minutes at a             standard pump speed (e.g., 200 μL/min).         -   e. The first source of magnetic flux 150 is engaged, and             then fluid from the tube containing the analyte is directed             into the inlet of the fluid chamber 160. Fluid is directed             into the inlet of the fluid chamber 160 to accumulate             magnetic beads bound with E. coli 0157:H7 on the at least             one surface 143 of the flexural plate wave device 160 until             a desired amount is attached to the at least one surface             143. In one embodiment, the desired amount is achieved when,             for example, there is a frequency shift in the output of the             flexural plate wave device 104 of about 4000 ppm.         -   f. The flow of fluid from the tube containing the analyte is             then discontinued. Fluid from the wash buffer tube is then             directed into the fluid chamber 160 to wash away             nonspecifically bound material (i.e., materials other             than 1) magnetic beads and 2) magnetic beads with bound             analyte).         -   g. The first source of magnetic flux 150 is then disengaged.         -   h. Initiate automatic wash protocol to remove any magnetic             beads or matrix components.         -   i. The final signal output by the flexural wave plate device             104 is then measured. The baseline signal is compared with             the final signal to determine the concentration of E. coli             0157:H7 in the analyte sample.

C. Detecting Prostate Specific Antigen (PSA) in Human Blood Serum Using, for Example, the Method Steps of FIG. 3.

-   -   1. Prepare an analyte sample containing human serum obtained by         centrifugation from a human blood sample.     -   2. Concentrate the analyte sample in solution by performing an         immunomagnetic separation. A variety of commercial instruments         (e.g., Pathatrix by Matrix Microsciences and Bead Retriever by         Dynal Biotech) or manual methods may be used to perform the         immunomagnetic separation. An exemplary manual method involves:         -   a. Resuspend magnetic beads coated with PSA antibody (e.g.,             Dynabeads anti-PSA, available from Dynal Biotech) until the             magnetic bead pellet in the bottom of the tube disappears.             Place a microcentrifuge tube in the rack (e.g., a Dynal             MPC-S) of a magnetic plate. Pipette 1-20 μL of magnetic bead             stock solution into the tube (the volume of magnetic bead             stock selected is based on desired final bead             concentration).         -   b. Add 1 mL of the analyte sample to the tube in the rack of             the magnetic plate and close the tube.         -   c. Invert the rack a few times. Incubate the solution in the             tube at room temperature for 10 to 60 minutes with gentle             continuous agitation to prevent magnetic beads from             settling.         -   d. Invert the rack several times to concentrate the magnetic             beads into a pellet on the side of the tube. Allow about 3             minutes for proper recovery.         -   e. Open the tube and carefully aspirate and discard the             sample supernatant as well as the remaining liquid in the             tube's cap.         -   f. Remove the magnetic plate.         -   g. Add 1 mL of wash buffer (PBS-Tween) to the tube. Close             the cap of the tube and invert the rack a few times to             resuspend the beads.         -   h. Repeat steps d-g twice.         -   i. Mix the contents of the tube briefly using a vortex             mixer.     -   3. Detection of PSA         -   a. Functionalize (similarly as described by the steps in A.3             and A.4) the surface 143 of a flexural plate wave device 104             with PSA antibody.         -   b. Place a first inlet hose into a tube containing standard             wash buffer (1× PBS with 0.05% Tween 20) and a second inlet             hose into the tube containing the magnetic beads bound with             PSA (prepared in B.2). The first inlet hose and second inlet             hose are joined by a t-joint so the two hoses are in fluid             communication and fluid from either the first inlet hose or             from the second inlet hose is directed to an inlet of the             fluid chamber 160. Each of the two hoses has a valve that is             capable of permitting or limiting the flow of fluid through             respective hoses.         -   c. Place a first outlet hose in fluid communication with an             outlet of the fluid chamber 160. Fluid from the first outlet             hose is collected in a waste collection bottle.         -   d. A baseline output signal is obtained using the flexural             plate wave device 104. The baseline signal is measured with             standard wash buffer flowing into the fluid chamber 160 and             out of the fluid chamber 160 for about 5 minutes at a             standard pump speed (e.g., 200 μL/min).         -   e. The first source of magnetic flux 150 is engaged, and             then fluid from the tube containing the analyte is directed             into the inlet of the fluid chamber 160. Fluid is directed             into the inlet of the fluid chamber 160 to accumulate             magnetic beads bound with PSA on the at least one surface             143 of the flexural plate wave device 104 until a desired             amount is attached to the at least one surface 143. In one             embodiment, the desired amount is achieved when, for             example, there is a frequency shift in the output of the             flexural plate wave device 104 of about 4000 ppm.         -   f. The flow of fluid from the tube containing the analyte is             then discontinued. Fluid from the wash buffer tube is then             directed into the fluid chamber 160 to wash away             nonspecifically bound material (i.e., materials other             than 1) magnetic beads and 2) magnetic beads with bound             analyte).         -   g. The first source of magnetic flux 150 is then disengaged.         -   h. Initiate automatic wash protocol to remove any magnetic             beads or matrix components.         -   i. The final signal output by the flexural wave plate device             104 is then measured. The baseline signal is compared with             the final signal to determine the concentration of PSA in             the analyte sample.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of the equivalency of the claims are therefore intended to be embraced therein. 

1. An apparatus for detection of an analyte, the apparatus comprising: a fluid chamber having at least one opening for fluid to enter; a flexural plate wave device defining at least a portion of at least one interior surface of the fluid chamber; a monitoring device to monitor at least one signal output by the flexural plate wave device; a plurality of magnetic particles coated with a capture agent having an affinity for the analyte; and a first source of magnetic flux that can be positioned external to the fluid chamber close to the interior surface of the fluid chamber to selectively attract the magnetic particles to the at least one interior surface of the fluid chamber.
 2. The apparatus of claim 1, further comprising a fluid control device adapted to change at least one of fluid flow, pressure or trajectory in the fluid chamber.
 3. The apparatus of claim 1, wherein the first source of magnetic flux is capable of being located external to the fluid chamber within 200 μm of the interior surface of the at least one interior surface of the fluid chamber.
 4. The apparatus of claim 1, further comprising a second source of magnetic flux.
 5. The apparatus of claim 1, further comprising a characterizing device to characterize a property of the magnetic particles attracted to the at least one interior surface.
 6. The apparatus of claim 5 wherein the property is selected from the group consisting of particle concentration, density, size and combinations thereof.
 7. The apparatus of claim 1, further comprising an identification device to track use of the apparatus.
 8. The apparatus of claim 1, further comprising a flow diverter to alter fluid flow in the fluid chamber.
 9. The apparatus of claim 1, wherein the flexural plate wave device comprises a plurality of interdigitated electrodes.
 10. The apparatus of claim 1, further comprising a filter wherein the fluid passes through the filter prior to entering the fluid chamber.
 11. A cartridge for a resonant device system, the cartridge comprising: a first fluid chamber having at least one opening for fluid to enter; a flexural plate wave device defining at least one interior surface of the fluid chamber; and a first source of magnetic flux that can be positioned external to the fluid chamber close to the interior surface of the fluid chamber to selectively attract magnetic particles to the at least one interior surface of the first fluid chamber.
 12. The cartridge of claim 11, further comprising a second fluid chamber comprising at least one outlet in fluid communication with the at least one opening of the first fluid chamber.
 13. The cartridge of claim 12, wherein the plurality of magnetic particles are initially disposed within the second fluid chamber.
 14. The cartridge of claim 13, further comprising a second device to combine at least some of the plurality of magnetic particles with an analyte located within a fluid.
 15. The cartridge of claim 11, further comprising a second source of magnetic flux.
 16. The cartridge of claim 11, further comprising a characterizing device to characterize a property of the magnetic particles attracted to the at least one interior surface.
 17. The cartridge of claim 11, further comprising an identification device to track use of the cartridge.
 18. The cartridge of claim 11, further comprising a flow diverter to alter fluid flow in first fluid chamber.
 19. The cartridge of claim 11 wherein the first source of magnetic flux is capable of being located external to the first fluid chamber within 200 μm of the at least one interior surface of the first fluid chamber.
 20. The cartridge of claim 11, further comprising a filter wherein the fluid passes through the filter prior to entering the first fluid chamber.
 21. The cartridge of claim 13, wherein the second fluid chamber comprises at least one inlet in fluid communication with an outlet of the first fluid chamber. 22-43. (canceled)
 44. The apparatus of claim 1, wherein the first source of magnetic flux is retracted away from the flexural plate wave device to substantially reduce magnetic fields in the vicinity of the at lest one surface of the fluid chamber.
 45. The apparatus of claim 44, wherein the monitoring device monitors the at least one signal output by the flexural plate wave device while the fluid is flowing through the apparatus.
 46. The apparatus of claim 45, wherein the flow rate of the fluid is increased.
 47. The apparatus of claim 1, wherein the first source of magnetic flux is positioned within 100 μm of the interior surface of the fluid chamber.
 48. The apparatus of claim 11, wherein the first source of magnetic flux is positioned within 100 μm of the interior surface of the fluid chamber. 